Calibrating a Refractometer which one is wrong???

[EnderTurtle]

I'm trying to calibrate my refractomer. Went to the Bay Area Reefers workshop recently and received a refractometer standard solution. The specific gravity of the solution says its 1.026 according to the label.

I typically calibrate my refractometer with RO water which i buy from a water vending machine outside of Rite Aide.

When I calibrate my refractometer using the 1.026 standard and then I read the salinity of my RO water it puts the specific gravity BELOW 0.

So is the standard incorrect? Or is the specific gravity of my RO water really below zero?

It seems more likely that the standard is actually saltier than it states. Meaning it's 1.027 if I calibrate using RO water.

[Secondsbest]

I've always read its best to use a standard for calibration. Calibrating with RO may not be as accurate.

[hogfanreefer]

Originally Posted by EnderTurtle

I'm trying to calibrate my refractomer. Went to the Bay Area Reefers workshop recently and received a refractometer standard solution. The specific gravity of the solution says its 1.026 according to the label.

I typically calibrate my refractometer with RO water which i buy from a water vending machine outside of Rite Aide.

When I calibrate my refractometer using the 1.026 standard and then I read the salinity of my RO water it puts the specific gravity BELOW 0.

So is the standard incorrect? Or is the specific gravity of my RO water really below zero?

It seems more likely that the standard is actually saltier than it states. Meaning it's 1.027 if I calibrate using RO water.

Neither. You want to calibrate the refractometer in the range of the solution it's being used to test. Always use a standard solution that's near 35 ppt.

PS I don't think it's possible for water to have a SG below zero by definition.

[accordsirh22]

thats why you are not supposed to use RO to calibrate them.

[EnderTurtle]

Well the Refractomer Salinity Standard is at 1.026 SG and 35ppt.

So you're saying trust the Standard? Which would result in a reading of below Zero when measuring the SG of RO freshwater.

[hogfanreefer]

Originally Posted by EnderTurtle

Well the Refractomer Salinity Standard is at 1.026 SG and 35ppt.

So you're saying trust the Standard? Which would result in a reading of below Zero when measuring the SG of RO freshwater.

You're not going to be using it to measure freshwater so you calibrate with what you are testing. I guess if you want to test fresh water you should calibrate with RO/DI water.

[tmz]

Any variation in the scale of the instrument is magnified at points further away from the range measured. So, calibrate with 53 ms solution( 1.264 sg).

[eacosta]

I calibrate with solution, however never hurts to double check it. Take the solution to an LFS and see what they read it at. My distilled water that i used to use to calibrate with reads at 1.0 sg (0 ppt) when I calibrate with 1.026 (35ppt) solution.

[Randy Holmes-Farley]

It is always correct to calibrate reef refractometers using a proper 35 ppt (1.0264) standard to make measurements of seawater.

It can be inherently inaccurate to calibrate at 0 ppt (RO water) because these are often brine refractometers and not actually seawater refractometers. Since that is often the case, if you calibrate at 0 ppt, you will read 35 ppt incorrectly even if the refractometers is perfectly made and working perfectly.

I discuss that here:

Refractometers and Salinity Measurement
http://reefkeeping.com/issues/2006-12/rhf/index.php

from it:

Imperfect Refractometer Use: Scale Misunderstanding and Salt Refractometers
Refractometers can lead to incorrect readings in additional ways and, again, these issues abound for reef aquarists. One is that many refractometers are intended to measure sodium chloride solutions, not seawater. These are often called salt or brine refractometers. Despite the scale reading in ppt (‰) or specific gravity, they are not intended to be used for seawater. Unfortunately, many refractometers used by aquarists fall into this category. In fact, very few refractometers used by hobbyists are true seawater refractometers.

Fortunately for aquarists, the differences between a salt refractometer and a seawater refractometer are not too large. A 35 ppt sodium chloride solution (3.5 weight percent sodium chloride in water) has the same refractive index as a 33.3 ppt seawater solution, so the error in using a perfectly calibrated salt refractometer is about 1.7 ppt, or 5% of the total salinity. This error is significant, in my opinion, but not usually enough to cause a reef aquarium to fail, assuming the aquarist has targeted an appropriate salinity in the first place. Figure 23 shows the relationship between a perfectly calibrated and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when the units are reported in salinity. This figure shows the measured salinity reading for seawater being about 1.7 ppt higher than it really is.

It turns out that this is a slope miscalibration in the sense that a perfectly made sodium chloride refractometer necessarily has a different relationship between refractive index and salinity than does seawater. This type of problem with a refractometer IS NOT at all corrected by calibrating it with pure freshwater. If you have this type of refractometer, and it was perfectly made and calibrated in freshwater, it will ALWAYS read seawater to be higher in salinity than it actually is (misreporting an actual 33.3 ppt to be 35 ppt).

Even more confusing, but perhaps a bit less of a problem in terms of the error's magnitude, salt refractometers sometimes read in specific gravity. But that value is specific gravity of a sodium chloride solution with the measured refractive index, not seawater with that refractive index. A sodium chloride solution with the same refractive index as 35 ppt seawater (which turns out to be 36.5 ppt sodium chloride) has a specific gravity matching 34.3 ppt seawater. So this type of refractometer, when perfectly calibrated, will read the specific gravity of 35 ppt seawater to be a bit low, at 1.0261 instead of about 1.0264. That error (reading 0.0003 or so too low) is, however, probably less than most reef aquarists are concerned with. Figure 24 shows the relationship between a perfectly calibrated and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when the units are reported in specific gravity. This figure shows the measured salinity reading for seawater being about 0.0003 lower than it really is.

Regardless of a salt refractometer's scale reading (ppt or specific gravity), aquarists can get around this problem by calibrating this type of refractometer in a seawater standard (see below). Because that type of calibration also gets around important manufacturing errors (slope calibration defects due to the scale being the wrong dimensions), it solves both problems at once.

[Mrramsey]

Mine specifically says Not to calibrate with RO water. I use the 35ppt solution.

[Randy Holmes-Farley]

Mine specifically says Not to calibrate with RO water.

I'm glad they finally are starting to do that. It's been a lengthy battle.

[fishman393]

Just got mine today Milwaukee refractometer. Try to calibrate it with the solution and reads hi. I then used to water and reads 0. Then tested solution and reads 1.027 and tank is at 1.020 I'm stuck.

[Randy Holmes-Farley]

Try to calibrate it with the solution and reads hi.

What reads high? If the calibration solution reads 1.027 after calibration, that's close, but a bit off (should be 1.0264, calibrate again).

If the tank water reads 1.020 after that calibration, it probably really is about 1.0206, which is pretty low, IMO.

[disc1]

Somebody check me on this, I may be wrong. I think the digital Milwaukee refracs are programmed to be calibrated to 0 with RODI. I know that is against every advise you'll get here, but if that's the way they programmed it then that's the only way you can do it.

If that is indeed true, then I would still take a 35ppt calibrant and do just like Randy just did. Check the calibration and fix it with math if it needs it.

[cangrejo]

Feel smarter already

[Randy Holmes-Farley]

I read that some manufacturers claim they can be calibrated at 0 ppt, which would be correct if they are a true seawater refractometer. There used to be nobne in the hobby, but now there may be some.

I also heard, like David mentions, that some claim they must be calibrated at 0 ppt, which cannot possibly be correct if the goal is reading seawater (assuming it is not an electronic device that forbids any other calibration).

How could you not calibrate a refractometer using the known solution that it is intended to read?

So my opinion is that it can never be incorrect to calibrate (or at the very least, check the calibration) using a good 35 ppt standard. It may also sometimes be correct to calibrate with 0 ppt, but you can always recheck that with the 35 ppt. If there is ever a discrepancy, pick the calibration that gets the right answer for the 35 ppt standard, no matter what reading that forces on pure fresh water.

[tmz]

I agree calibration at 1.026 is better than zero for measuring salt water in our tanks.

However, the digital read out Milwaukee and Hanah digital refractometers do not have a a calibration capability as we are used to for standard refractometers. I'm not sure that's what the OP has though.

These digital readout meters only have a zeroing function for calibration which is best served with deionized water.

Basically , you fill the small glass dish with deionized water and push the zero button to calibrate it . The light in it flashes and the screen reads 0.000. No other way to do it with these. Then dry it off ; fill it with a pipette full of sample water and press read and the sg is displayed on the screen to the 3rd decimal,eg 1.027.

The Milwaukee , I have does crosscheck closely with my old standard refractometer calibrated with 53 ms solution and the pinpoint conductivity meter, fwiw. I like it because it's an easy read for my old eyes.

[bnumair]

Originally Posted by disc1

Somebody check me on this, I may be wrong. I think the digital Milwaukee refracs are programmed to be calibrated to 0 with RODI. I know that is against every advise you'll get here, but if that's the way they programmed it then that's the only way you can do it.

If that is indeed true, then I would still take a 35ppt calibrant and do just like Randy just did. Check the calibration and fix it with math if it needs it.

i have the same equipment and there is only one way to calibrate it is through RO/DI water. Milwaukee meter has to be set zero with ro/di water and there is no way to calibrate it with 35ppt solution as its only got a ZERO button, ur suppose to hold it to 0 it out when u have ro/di water in the well.
Now i do check it after calibrating with the 35ppt solution and its usually dead on. One thing about there meters is that the solution and meter itself should be used inside the house within regular temp o of 70-90F range. My setup is in garage and during summer days it hits 140F in the garage and using this meter at those temps give a wrong reading so i bring the unit inside the house for it to cool off a bit. Also leaving the sample in the well for 2-3 min before taking a reading give a better result.

[Randy Holmes-Farley]

Thanks.

I agree that if a device physically can't be calibrated at 35 ppt for reasons relating to its design, it can (and should, IMO) be checked at 35 ppt.

If you get a value different than 35 ppt or 1.0264, then just take the difference from that reading and add or subtract it from all the values you get using it.

[fishman393]

I did check mine with to water then retested the 35ppt and it is dead on now. My tank was at 1.020. When I check with the hydrometer it reads 1.028. So crazy glad I got this digital reader. Thank guys for the input. Time to bring up my salt lvl in tank.

[dkeller_nc]

Just a note about the Milwaukee digital refractometers - they are indeed calibrated to produce correct specific gravity results with seawater instead of brine. And the typical user can only zero the instrument with purified water.

That said, I've worked on 3 of these for other folks, and they're typically dead-nuts "on" with a lab-prepared standard of either sodium chloride or IO salt mix.

It is possible to set both the zero and span of these instruments, but it requires opening them and messing with the circuit board, which will void the warranty, so not something advisable for an average reef tank hobbyist.

Here are a couple of tips for those new to the instrument:

The temperature calibration is quite good, but it takes about 10 seconds for the thermal mass of the sample well and the sample to come to thermal equilibrium. If the instrument is very cold (such as when stored in a kitchen cabinet in the winter), it may take two or three sample additions/removals to get the sample well close to the tank temp.

Repeated use with tank water will coat the optical window and lead to errors typically in the 0.001 - 0.002 specific gravity range. This is why it's a good idea to rinse the sample well with a couple of pipettes full of RODI after use, and occasionally wipe it clean with either lint-free tissue or a clean microfiber cloth. Despite the directions to the contrary, I recommend against folks wiping the optical window with each use, as it will buildup micro-fine scratches over time which will definitely affect its accuracy.

There are two major issues that I've seen with these instruments. The first is easy to correct - a low 9V battery. Depending on its state of discharge, a low 9V battery can cause the instrument to give the error messages "Hi" or "Air". The second is folks that store the instrument in their sump cabinet. The air inside the instrument exchanges with the air in the cabinet, which is near 100% humidity and >75 deg F. When the instrument is pulled out and set on the counter during the cool periods of the year, water condenses on the electronics inside. One individual's circuit board was so corroded from this practice that it was beyond repair.

[dkeller_nc]

Originally Posted by fishman393

I did check mine with to water then retested the 35ppt and it is dead on now. My tank was at 1.020. When I check with the hydrometer it reads 1.028. So crazy glad I got this digital reader. Thank guys for the input. Time to bring up my salt lvl in tank.

Careful! Don't add salt directly to the tank or sump, nor change the specific gravity more than 0.002 per 24 hour period (assuming that your tank is populated).

The best way to increase the salinity slowly is to add a little bit of normal fresh seawater, and let the tank evaporate down to either the ATO's trip point, or your "make up" mark on the sump/tank if you do manual top-offs.

[arinrb]

As a bit of a side note, RO and RODI are not the same; it would be best to use RODI in your tank.

I heat measuring salt, I stress about it more than anything else in this hobby.

[tmz]

Actually. I meant to say distilled water in my post. That's what I use since my ro/di in storage isn't always completely pure. .

[fishman393]

It's a new tank no live stock yet. Thanks for all the info. But I did mess up and add a little salt to the sump mixed with to water. Poured it in the sock. Seems like it worked for me I'm going to let it run and double check in the morning.